Torpedoes and submarines are spool shaped vessels that are propelled below or at the surface by means of propeller drive. To avoid the torpedoes from rotating around its longitudinal axis, they normally need some device for recovering the rotation energy created by the propeller.
Counter rotating propellers have been used to provide propulsion without generation of torque, see e.g. SE 40408. However, the solution is not always acceptable under military circumstances, where nowadays both high speed and silent propulsion are given high priority.
A propulsion system, designed according to the state of art generally designated "pumpjet" system, for torpedoes or submarines is designed according to the introduction of the description, whereby the shroud is supported by a number of supports that extend in different radial directions from the vessel's body. The stator on its part is fixedly mounted in the rear part of the shroud, downstream the rotor.
The supports placed in front of the rotor create interferences in the flow to the rotor. These interferences can cause variations in stress on the rotor blades. The variations in stress produce a striking increase in noise radiating out from the pumpjet system arrangement. Besides, the interferences can cause hydraulic cavitation on the rotor blades, which forms a very strong source of noise, and can damage the surfaces of each blade.